Radio Frequency Identification (RFID) is utilized in a variety of applications with RFID readers communicating with RFID tags for purposes of identification, location, tracking, and the like. In an exemplary RFID application, an RFID reader may be mounted overhead (e.g., ceiling mounted) relative to a plurality of RFID tags, such as in a retail environment, a factory environment, a warehouse environment, etc. The overhead configuration offers several advantages such as fewer physical obstructions, ease of access to wiring in a ceiling, tamper resistance, safety, and the like. However, conventional overhead antenna configurations have disadvantages.
For example, it is desirable for an overhead RFID reader to be able to passively read all the RFID tags in the environment. However, it should be recognized that the tags and their antennas may have all different orientations, depending on how they are placed or stored in the environment. Optimally, a tag that is placed horizontally is best read by an RFID reader with horizontal polarization, and a tag that is placed vertically is best read by an RFID reader with vertical polarization. Of course, such perfect alignment is rarely achieved.
One solution to this random tag orientation is to provide cross-polarization, which provides a vertically polarized antenna and a horizontally polarized antenna, with overlapping RF coverage between antennas, in an RFID reader, where the RFID reader can switch between the antennas. One example of such an antenna arrangement is a cross-dipole where two dipole antennas are arranged at 90 degrees to each other making a “+” shape and are both fed in the center. Cross-polarization can read tags that are at either orientation and also tags angled between vertical and horizontal orientations, but with reduced gain. However, since each cross-polarized reader uses two antennas, the number of antennas ports required is doubled and the physical size of the solution is larger. In addition, such cross-polarized readers are not omnidirectional and do not have a 360 degree beamwidth, requiring several RFID readers to be deployed to cover the entire environment. Another solution to the problem is to provide circular polarization that can read tags at any orientation. However, such circular polarized readers need to be larger in size in order to maintain the same gain as their linearly polarized equivalent (i.e. 6 dB linear=9 dB circular). In addition, circular polarized readers are not omnidirectional and do not have a 360 degree beamwidth, requiring several RFID readers to be deployed to cover the entire environment.
Regarding coverage area, RFID ceiling reader antennas can be oriented in one of three ways—parallel, normal, or angular to the ceiling. As examples, when a slot antenna, a patch antenna, or a loop antenna is mounted parallel to the ceiling or a dipole antenna, or a Yagi antenna is mounted normal to the ceiling, the peak gain is at bore sight, with the main lobe of the antenna radiation directed perpendicular to the ceiling; much of the RF energy is therefore directed straight down to the floor/ground. In the angular mounted configuration, the angle of mount is selected to direct the main radiation lobe of the radiation pattern to a target of interest. A problem in these above scenarios is that, as we move away from the main lobe of the radiation pattern, the gain of the antenna begins to drop. For RFID applications, this situation results in a requirement to install multiple RFID readers with antennas aimed at various angles to get a consistent and a high percentage of RFID read coverage. However, the use of multiple readers not only drives the installation cost up but also does not result in a high percentage of tag reads in areas where the antenna gain falls from its peak. It is therefore very important to simplify and minimize the size, weight, and cost of the reader without compromising RF performance.
Accordingly, there is a need for an RFID antenna apparatus and method that overcome the aforementioned limitations. It would be beneficial to provide this overhead system in a small and lightweight arrangement while minimizing the number of RFID reader systems (especially ceiling mounted) installed in a particular environment, and maintaining/increasing overall read accuracy and read percentages.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.